May be Myanmar and Bangladesh should do what Laos did. China funded and built a dual use rail system for Laos that allows passenger trains to travel at 160 kmph (99 mph) and regular freight/passenger train at 100 kmph. That means that when the full SE Asia HSR system is completed, a HSR train can travel from China at full speed (300+ kmph) inside China, slow down a bit in Laos, and then resume full speed once it cross into Thailand and then all the way to Singapore without changing train.
Since its opening of this railway in 2021 December, the utilization of this railway is beyond expectation even though passenger traffic is still limited due to Covid-19. The Thais are a heavy user of this railway to ship perishable goods to China. The economic impact to Laos is already clearly visible and the ROI is looking more and more attractive.
If Bangladesh and Myanmar implement such system, then it is quite possible we can see in the future, Indian HSR train will be able to go all the way to Beijing or Singapore and vice versa. That will be great for the continental S, SE and E Asia. But a lot has to happen economically and geopolitically before then.